Counting the Blessings of Biodiversity
Species extinction ranks high as a concern among those interested in how ecosystems function. According to the fossil record, several thousand plants and animals have disappeared over the last ten million years; during the time dinosaurs were alive, one species disappeared about every one to ten thousand years. But as the human population has grown, so has the rate of extinctionresearchers now conservatively estimate that species are dying out at the dramatic rate of one a day.
The assumption, of course, is that this can't be good. More than a century ago, Charles Darwin first suggested that more species would make an ecosystem more productive. But researchers have struggled to test the notion rigorously, not just in the lab but in the field.
It wasn't until 1996 that anyone had real evidence that biodiversitysheer numbers of different speciesis critical to the planet's well-being.
In an experiment that other ecologists have described as "brilliant" and "a first," University of Minnesota ecologist David Tilman and other researchers at the Cedar Creek Natural History Areaan NSF-funded LTER site since 1982demonstrated that plant communities with the greatest biodiversity yielded the greatest total plant growth year to year. These plant communities also were much more likely to hang on to essential nutrients that might otherwise have been leached from the soil.
Tilman's team approached the problem by constructing 147 miniature prairies within a section of the 5,500-acre experimental reserve at Cedar Creek, and planting each one with anywhere from one to twenty-four species. The burning, plowing, and planting were done by the spring of 1994. Then the researchers sat back to see which plots would end up doing best.
Actually, no one sat much. The researchers, aided by an army of undergraduates, have toiled ever since to meticulously weed the 100-square-foot plots of anything that didn't belong to what each plot was designed to contain, be it brown-eyed susans, bunch clover, or yarrow. A critical aspect of the study was that researchers randomly selected which species went into which plots. This kept the focus on the number rather than the type of species.
Why do more species make for a merrier ecosystem? Tilman has found that a diverse plant community uses the available energy resources more efficiently.
"Each species differs from others in a variety of traits," says Tilman. "Some have high water requirements and grow well during the cool part of the year. Others grow well when it's really warm and dry. Each one in the system does what it's good at, if you will, but there's always something left to be done." That is, conditions that are less than hospitable to some species will be readily exploited by others, leading to more lush growth overall. These processes, says Tilman, also explain why so many species can coexist in nature.
"It wasn't until we knew how rapidly species were going extinct that this issue really came to the forefront," says Tilman. Still, more work needs to be done before biodiversity's role in a healthy ecosystem can be unequivocally celebrated. That's why Tilman and other Cedar Creek researchers have added a second experiment to the mix, this time using more than three hundred bigger plots, each about the size of an average suburban backyard. The extra area should allow a better understanding of how, for example, plots with different numbers of species handle insects and disease.
"Nobody's ever done what they've done," says Samuel McNaughton, an ecosystem ecologist at Syracuse University in New York. "It's an enormous amount of work. Tilman would not have been able to do this without NSF funding through the LTER program."